1. Field of the Invention
The present invention relates to water-soluble fibers comprising a polyvinyl alcohol ("PVA") and having excellent dimensional stability. More specifically, the present invention relates to water soluble PVA fibers which, while being readily soluble in hot water at a temperature up to 100.degree. C., shrink only to a small extent under high humidities, as well as upon dissolution, and have high tensile strength and small ash content. These fibers, having the above features, have very good handleability and give high-quality finished products and are hence suitably used for chemical lace based fabrics, blend yarns with wool, flax or ramie and like items.
2. Description of the Prior Art
Known water soluble fibers include PVA-based fibers, cellulose-based fibers such as carboxymethylcellulose fiber, polyalginic acid fiber, polylactic acid fiber, polyalkylene oxide fibers and the like, and are suitably used utilizing their features. Among these water soluble fibers, PVA-based fibers are used most widely because of their high tensile strength.
Various water soluble PVA fibers have been proposed by, for example, Japanese Patent Publication Nos. 8992/1968 and 10174/1978 and Japanese Patent Application Laid-open Nos. 199408/1991, 28408/1987, 86503/1993, 45424/1978 and 229805/1989.
Of this above literature, Japanese Patent Publication No. 8992/1968 describes a process for producing a water soluble fiber which comprises conducting dry spinning of a high-concentration aqueous PVA solution. However, the fiber obtained by this process has a large shrinkage upon dissolution in water of 30% and hence chemical lace base fabrics utilizing this fiber shrink, when being dissolved in with water, to a large extent, thereby deforming the lace pattern embroidered thereon. Consequently, such base fabrics are not usable for preparing high-quality laces having fine patterns.
Japanese Patent Publication No. 10174/1978 describes a process for producing a fiber which is soluble in low temperature water, which comprises using a carboxyl group-modified PVA as raw material. However, the fiber obtained by this process has the drawback of shrinking to a large extent when absorbing moisture, when allowed to stand under high humidities. The fiber as well as finished products obtained therefrom must therefore be stored under a specific atmosphere with controlled, low-humidity.
Japanese Patent Application Laid-open No. 199408/1992 describes a process for producing a water-soluble fiber from a PVA which has low degree of a polymerization of not more than 500, in order to decrease the shrinkage of the fiber upon dissolution in water. The PVA used in this process, having a low degree of polymerization, can only give fibers having a very low strength of less than 3 g/d. Furthermore, the obtained fiber contains boric acid or a borate (in particular, low temperature soluble types of this fiber contains a large amount of boric acid or a borate), thereby causing the effluent water used for dissolving the fiber to contain a large amount of boric acid, the treatment of which requires a special process and apparatus.
Japanese Patent Application Laid-open No. 28408/1987 describes, an improvement in the spinnability of a PVA having a low degree of polymerization, which should give a fiber having small-shrinkage solubility, by employing a technique which comprises adding to the PVA a small amount of another PVA having a high degree of polymerization, thereby obtaining a PVA having both good spinnability and small-shrinkage solubility. Even with the fibers obtained by this technique, mainly containing the low-polymerization-degree PVA, a small shrinkage type of not more than 20% has a low strength of not more than 3 g/d. Such a water soluble fiber with low tensile strength has poor processability during knitting or weaving or during nonwoven manufacturing. In addition, the fiber readily breaks when handled by embroidery needles upon embroidery of chemical lace on base fabrics made therefrom. Fine embroidery is impossible with such base fabrics.
Japanese Patent Application Laid-open No. 86503/1993 describes a technique having the same object as that of the present invention which is to improve the dimensional stability of a water soluble fiber under high-humidity conditions. However, the fiber actually obtained by the technique has a considerably large shrinkage, at a RH of, 80% of at least 3.5%. The fiber, like that obtained by the above process disclosed in Japanese Patent Publication No. 10174/1978, has a very serious problem in that fibers or articles processed therefrom must be stored under low-humidity conditions.
Japanese Patent Application Laid-open No. 45424/1978 describes a process for producing a water soluble fiber having a small shrinkage in water at not more than 50.degree. C., which comprises wet spinning an aqueous solution of a PVA having a low saponification degree into a concentrated aqueous solution of a salt such as sodium sulfate and then drawing the obtained as-spun fiber in a low draw ratio. However, the fiber obtained by this process, which uses a high concentration aqueous salt solution, as a coagulating bath, contains a large amount of the salt adhering thereto. Washing with water then becomes necessary to remove this salt from the fiber, but complete washing is very difficult, since the fiber itself is water soluble. Thorough washing would dissolve the fiber surface and cause the fibers to stick together, so that fibers that have small ash content and do not stick with each other cannot be obtained. Besides, the fiber obtained by this process, while having a small shrinkage in water at not more than 50.degree. C., shows a large shrinkage at a higher temperature just before dissolution, and therefore has poor dimensional stability.
Japanese Patent Application Laid-open No. 229805/1989 describes a process for producing a water soluble PVA fiber having high tensile strength, which comprises dry-jet-wet spinning a solution of a PVA having a low saponification degree in an organic solvent such as dimethyl sulfoxide (hereinafter referred to as "DMSO") into a solidifying bath such as methanol having a solidifying function and then drawing the solidified fiber in a high draw ratio. However, the fiber obtained by this process, in which the strain due to the high-ratio drawing still remains, shows, when kept under high humidities, a large shrinkage due to moisture absorption and also shrinks to a large extent upon water dissolution, and thus has poor dimensional stability. The object of the technique described in this laid-open application is not to provide a fiber having good dimensional stability but, rather, from the description that the fiber is suitably used for preventing side leaks of disposable diapers, to provide a fiber having a very high shrinkage when wetted.
In the field of chemical lace base fabrics, it is required that fibers constituting the fabrics be soluble in low-temperature water. However, such low-temperature soluble fibers shrink by absorption of moisture in the air and should therefore be stored in a low-humidity atmosphere, which fact makes storage and control of the fibers and fabrics made therefrom very difficult. If water soluble fibers have a low tensile strength, they readily break by action of the needle upon embroidery on the base fabric made therefrom, whereby fine-design embroidery, i.e. high-grade embroidery cannot be obtained. Furthermore, with water soluble fibers shrinking to a large extent upon dissolution, the obtained embroidered patterns deform at the same time, so that high-grade embroidery cannot be obtained.
As another end-use of water soluble fibers is in a process which comprises preparing blended yarns or blend twisted yarns of water soluble fibers with wool, flax or fibers, processing the obtained yarns into woven or knit fabrics and then dissolving the water soluble fiber component thereby obtaining fabrics having a unique hand or drape or improving the processability in the steps of spinning through weaving or knitting. If the water soluble fiber used for this purpose shrinks upon dissolution, which increases the apparent density of the structure containing them, their complete dissolution will become difficult. If the water soluble fibers have a low tensile strength, they tend to break during spinning through weaving or knitting, thus showing poor processability. Where the water soluble fibers have high ash content because they carry on their surface salts, boric acid or the like, such salts readily adhere to weaving or knitting machines or chemical lace manufacturing machine, thereby causing the machines to rust. Furthermore, in this case, the water used for the dissolution necessarily contains chemicals such as boric acid, which require complex post-treatment of the effluent water.
However, no known techniques have, as described above, succeeded in giving a water soluble fiber that shrinks only to a small extent upon dissolution in water and has good dimensional stability under high humidities, almost no ash content and high tensile strength.